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A North Korean Ilyushin-62 plane pictured at South Korea's Incheon airport in February after arriving with a high-level delegation (AFP Photo/-)

When North Korean leader Kim Jong Un hitched a ride on an Air China plane to his summit with Donald Trump in Singapore, the signal was clear: Beijing remains Pyongyang's diplomatic wingman.

Kim had never flown so far from Pyongyang since coming to power in December 2011, and having China give him a lift shows that the Cold War-era allies still need each other despite recent tensions.

North Korea's ruling party newspaper pointedly published photos of Kim boarding the Air China Boeing 747, a surprising display for a country that boasts its "juche", or self-reliance, ideology, according to analysts.

"It may be a practical reason, but at the same time it was a symbolic move to show its people that China has North Korea's back and China could be there for the North if the whole denuclearisation process with the US doesn't work," Koh Yu-hwan, professor of North Korean studies at Seoul's Dongguk University, told AFP.

The crew used some sleight of hand en route to conceal the presence of their VIP passenger on Sunday, abruptly changing the aircraft's callsign in midair over Beijing before continuing southward across China, according to flight tracking website Flightradar24.

Chinese foreign ministry spokesman Geng Shuang said Monday that North Korea had requested the plane and that "China's civil airline offered the relevant service".

Analysts said Beijing also had Kim's personal safety in mind given North Korea's ageing fleet. His father and predecessor Kim Jong Il was famously afraid of flying and only travelled by train.

Kim has his own private jet which he used to visit President Xi Jinping in nearby northeast China in May.

His sister and close aide Kim Yo Jong is believed to have used it to travel separately to Singapore, echoing the US practice of presidents and vice-presidents generally never flying together on Air Force One -- or any other aircraft -- to ensure one of them survives in the event of disaster.

But the US President's Boeing 747 is a far cry from Kim's aircraft, a Soviet-made Ilyushin-62 which wags have dubbed Air Force Un.

- 'Remain vigilant' -

China and North Korea fought side-by-side against US-led UN forces and South Korea in the 1950-1953 Korean War.

But relations have been rockier in recent years, with China, the North's main economic lifeline, supporting UN sanctions to punish it for its nuclear and missile tests.

But amid the flurry of diplomacy on the peninsula triggered by the Winter Olympics, Kim reached out to his estranged protector, appearing suddenly in Beijing in March to finally pay his respects to Xi Jinping -- six years after inheriting power.

The pair met again less than two months later.

Kim's grandfather Kim Il Sung was adept at playing off Communist rivals Beijing and Moscow against each other during the Cold War to extract concessions from both.

"Pyongyang may think that now it can use the same tactic over the rivalry between China and the US to its own advantage," Koh said.

At the same time, he said, Kim will want to strengthen his relationships with powers such as China and Russia to protect himself if the North does start dismantling the nuclear arsenal that it has long called its "treasured sword".

That desire will only have been strengthened by US references to the "Libya model" of nuclear disarmament -- leader Moamer Kadhafi was later deposed and killed in a Western-backed uprising -- and by Trump's unilateral withdrawal from the Iran nuclear deal.

Beijing, for its part, is intent on keeping the North -- whose existence as a buffer state keeps US troops well away from China's borders -- firmly within its sphere of influence.

"China might still need to be vigilant about possible subterfuge of South Korea and the US," Lu Chao, a North Korea expert at the Liaoning Academy of Social Sciences, told the state-run Global Times newspaper.

He warned that if Washington, Seoul and Pyongyang sign a peace treaty to formally end the state of war on the peninsula without Beijing, "China has the right to nullify it".

Now China will be paying close attention to what transpires in Kim's meeting with Trump.

"Any time two volatile and unpredictable guys get together to discuss an issue that is of central national security importance to you, that's a cause for anxiety," said Jonathan Sullivan, director of the China Policy Institute at the University of Nottingham.